I found this old entry for a design challenge I wrote back in 2008 for another website. I thought it was a pretty cool idea, so I decided to post it here.
The challenge was to design an NPC for an MMORPG, I believe.

Description
The Mimicry Spectre will mimic the appearance of another player's avatar (NOT the avatar of the player himself). It only appears when there are currently no avatars other than the player's in the field of view. Its appearance is random, yet it does require that another player is nearby (although not in the field of view). It is preferred that the avatar to be mimicked was recently seen by the player.
The Spectre will look completely identical to the avatar being mimicked, including a mimicked name tag. Its true name will only be revealed when a specific term has been met with. (See below.) Its mimicry will even go as far as repeating the words that are uttered by the imitated avatar. This will oft look out of place, and as such is an important cue for players to realize this is no ordinary player avatar.
The behavior of the Spectre is inconspicuous at first, but will turn aggressive as the player gets close to the Spectre. As long as "the term" hasn't been met, the Spectre is invincible: all attacks (physical, magical or otherwise) will pass right through it. When the Spectre is within melee attack range of the player, it will deal a single strike (with no weapon in particular). When hit, the Spectre image will fade out, and will not re-appear. The hit deals massive damage to the player.
The only way for a Mimicry Spectre to be defeated, is to get the mimicked player into the field of view, together with the Spectre. (As our game is 2D, I expect there won't be a 3D camera, and the camera will simply be fixed or semi-fixed on the player avatar.) The Spectre will not typically follow the player for long stretches, so the preferred way to meet this term is to shout the mimicked player’s name (at that time the same name carried by the Spectre), and hope he or she will respond to the call. The moment both Spectre and mimicked player are in the player’s field of view, the Spectre will freeze, its motions halt and both a visual effect and the changing of his name (to “Mimicry Spectre”) further informs players of the deception. During this time, either player has a brief moment to deal a single defeating blow to the Spectre. Wait too long however, and the Spectre will fade, taking its spoils with it. Defeating the foe will leave behind spoils for both players, although slightly more for the player that delivered the coup de grace.
Mimicy Spectre can appear multiple times throughout the game and becomes less common as the game progresses.

Additional Comment
The Mimicry Spectre will encourage newer players to communicate with others, through experience and reward.